The secret to effortless vibrato, shifting, high position work, and a lot of other things is using your back to lift your arms. If you're playing the viola, and especially if you are slight of build, please do this exercise to avoid injury.

As usual, do fewer repetitions than your limit. You want it to not feel like work, so that you'll be likely to do it the next day. However, you have to do at least 1! :-)

I'll post a video of this as soon as I get a chance.

1. Stand without holding your instrument or bow, in good posture. Arms at your sides.
2. Let your shoulders rotate back and let your shoulder blades sink down in back, causing your arms to lift in front of you, sticking straight out in front.
3. Feel your shoulder blades slightly move towards each other as you swing your arms out to the sides, palms down.
4. Flip palms up
5. Stretch your hands away from you as much as possible and feel the muscles under your arms and around your shoulder blades as you press your arms upwards, ending with them straight up in the air over your head, palms facing towards each other.
6. Again, stretch your hands as high as you can as you "dive" back, returning your arms to the side, palms up. As your arms get closer to horizontal, feel your shoulder blades sinking down into their "slots".
7. Flip palms down
8. Slowly swing arms to front
9. Return arms to your sides.

Try doing this while continuously moving your knees, to keep your neck relaxed.

The most important parts of this exercise are using your back and underarms to raise your arms above your head, and letting the shoulder blades sink as your arms come back down.